Waveguide apparatus for microwave drying of materials

ABSTRACT

A 4-PORT HYBIRD COUPLING A MICROWAVE GENERATOR TO A SET OF WAVEGUIDES WHICH HAVE SLOTS THEREIN THROUGH WHICH LUMBER CAN TRAVEL AT AN ANGLE OF LESS THAN 90* TO THE LONGITUDINAL CENTERLINES OF THE WAVEGUIDES.

L. ADMIRAAL Jan. 23, 1973 WAVEGUIDE APPARATUS FOR MICROWAVE DRYING OFMATERIALS Filed June 5 MICRO WAVE GENERATOR PRIOR ART PI P2 um PRIOR ARTMICROWAVE 28 GENERATOR INVENTOR LAMBERTUS ADMIRAAL KTYORNGYS UnitedStates Patent "ice US. Cl. 219-1055 7 Claims ABSTRACT OF THE DISCLOSUREA 4-port hybrid coupling a microwave generator to a set of waveguideswhich have slots therein through which lumber can travel at an angle ofless than 90 to the longitudinal centerlines of the waveguides.

This invention relates to waveguide apparatus for drying moisture-ladenmaterials, such as lumber, wood-based materials, bagasse, paper and thelike, by microwave energy.

Although many types of materials may be dried by the present apparatus,it is primarily designed for lumber and is so described herein forconvenience.

When microwave energy is directed at a piece of lumber located in theslots of a waveguide system and extending at right angles to saidWaveguide, a large percentage of the power is reflected, particularly ifthe wood is wet. This represents a waste of power, and the reflectedenergy is damaging to the microwave generator. Wet lumber can reflect upto 60% of the microwave energy, while dry lumber can reflect up to 20%.

This waste of power can be very costly, but what is more important,drying under these circumstances is not practical because of the dangerto the generator.

This problem is greatly reduced or eliminated by the present invention.This is accomplished by arranging the slots in the waveguide system sothat the lumber extends at an angle of less than 90 to the longitudinalcenterline of the waveguide system. In addition, a 4-port hybrid is usedto couple the microwave generator to the waveguide system. This systempreferably is in the form of a set of parallel waveguides each having alongitudinal centerline. The hybrid has the usual ports 1 and 4 at oneend, and ports 2 and 3 at its opposite end. The microwave generator iscoupled to port 1 of the hybrid, while the waveguides are coupledrespectively to ports 2 and 3 thereof. With this arrangement it ispossible (through initial adjustment of the above-mentioned angle) todirect the reflected power from the lumber faces towards port 4 of thehybride and therefore away from the microwave generator. Furthermore, itis possible to connect to port 4 a means for the absorption of thereflected microwave energy, but this angling of the lumber makes itpossible to couple a microwave reflector to port 4 in order to returnthe reflected power to the lumber and thereby utilize most of thegenerated microwave power.

This angular arrangement greatly reduces the amount of energy reflectedby the lumber. For example, with the lumber path arranged so that thelumber travels at an angle of about 39 to the longitudinal centerline ofthe waveguides, the reflected power, all of which travels to port 4, hasbeen reduced to approximately 7% for all conditions of wetness. With areflector at port 4 of the hybrid, the original reflected energy isdirected back to the lumber and 7% of this or about .5% of the total isreflected again by the lumber. This reaches the microwave generator, butit is so low that it cannot cause any trouble.

Waveguide apparatus according to the present inven- 3,712,971 PatentedJan. 23, 1973 tion comprises a waveguide system adapted to be coupled toa microwave generator and having a centerline along which microwaveenergy from the generator travels, and opening means in said waveguidesystem through which lumber can pass to be exposed to said microwaveenergy, said opening means being arranged so that the path of travel ofthe lumber is at an angle of less than to said centerline. It ispreferable to provide stop means between the generator and the waveguidesystem for preventing microwave energy reflected by the lumber fromreaching the generator.

A preferred form of Waveguide apparatus according to this inventioncomprises a set of parallel waveguides having longitudinal centerlines,a microwave generator coupled to the waveguides, slots in the waveguidesthrough which lumber can pass to be exposed to microwave energy from thegenerator, these slots being arranged so that the path of travel of thelumber is at an angle of less than 90 to the centerlines of thewaveguides, and stop means between the generator and the waveguides forpreventing microwave energy reflected by the lumber from reaching thegenerator. The stop means is preferably a 4-port hybrid having the usualports 1, 2, 3 and 4. The generator is coupled to port 1, and thewaveguides are coupled to ports 2 and 3. If desired, microwave energyabsorption means can be coupled to the 4th port of the hybrid, but it ispreferable to couple a microwave reflector to said port so as to returnmicrowave energy which is reflected by the wood back to the waveguides.

An example of this invention is illustrated in the accompanyingdrawings, in which FIG. 1 diagrammatically illustrates a waveguide withlumber travelling therethrough at right angles to the centerlinethereof,

FIG. 2 is a side elevation of part of the waveguide of FIG. 1,

FIG. 3 diagrammatically illustrates waveguide apparatus in accordancewith this invention, and

FIG. 4 is a fragmentary isometric view of part of the apparatus of FIG.3.

Referring to FIGS. 1 and 2 of the drawings, 10 is a waveguide having amicrowave generator 12 coupled to an end thereof. Slots 13 in thewaveguide are arranged so that lumber 14 passing therethrough extends atan angle of 90 to the longitudinal center of the waveguide. microwaveenergy P1 is directed through the waveguide against an edge of lumber14. Reflected microwave energy P2 is directed back through the waveguideto the generator. P3 represents the microwave energy absorbed in thewood, and P4 is the energy that is transmitted through and beyond thewood.

The reflected power P2 travels back to the microwave generator where itcauses a temperature rise and also interfers with normal operation ofthe generator. This represents a great waste of power, and the generatorwill not function very long under these conditions. As stated above, upto 60% of the power can be reflected by wet lumber, and up to 20% by drylumber.

FIGS. 3 and 4 illustrate waveguide apparatus 20 incorporating thepresent invention. This apparatus includes a set of waveguides or arms23 and 24 extending parallel to each other, and a 4-port hybrid 26coupling these waveguides to a microwave generator .28. The waveguide 23has a general longitudinal centerline 30 which is parallel tolongitudinal centerline 31 of waveguide 24. Sections 23 and 24 of thewaveguide system have slots 35 and 36 therein arranged, see FIG. 4, sothat the path of travel of lumber 38 through these slots is at an angleof less than 90 to the centerlines 30 and 31, as indicated at 40 in FIG.3. Although any angle less than 90 can be used,

it has been found that the best results are obtained by using an angleof from 35 to 55".

Hybrid 26 is formed with a central section 44, and a first pair of arms46 and 47 project from one end of said central section at an agle toeach other and which are coupled to the waveguides 23 and 24respectively. A second pair of arms 49 and 50 project from the oppositeend of the central section of the hybrid and at an angle to each other.These arms 46, 47, 49 and 50 constitute the ports 2, 3, 1 and 4,respectively, of the hybrid. The microwave generator 28 is coupled toarm 49 so that microwave energy therefrom is directed equally througharms 46 and 47 of the hybrid against the lumber 38 travelling along theinclined path through Waveguide slots 35 and 36. It is preferable toprovide a microwave reflector or reflection plate 53 at the end ofhybrid arm 50, although a suitable water load, indicated at 55, may beprovided instead.

Microwave energy or power P1 from generator 28 is divided in hybrid 26equally between arms 46 and 47 as P5 and P6. The energy is directedagainst the inclined edge of lumber 38. The crossing of the longitudinalcenterline of the hybrid from arm 49 to arm 47 gives an automatic 90phase shift to P6. The power P7 reflected by the lumber in waveguidesection 24 is divided in the hybrid, and P8, which is one half of P7,enters arm 50. The other half of P7 crosses the centerline and movestowards arm 49. This crossing of the centerline imparts an additionalphase shift of 90 to the power directed towards arm 49 so that it nowhas a 180 phase shift from the original power. At the same time,reflected power P9 in waveguide section 23 is divided in the hybrid,with half directed towards arm 49. As this is 180 out of phase with thepart of power P7 directed towards said arm, propagation in arm 49 isprohibited. The only place to go for this energy is arm 50 where it willreinforce the half being directed to arm 50 since they are in phase witheach other. The remainder of P9 which crosses over in the center of thehybrid enters arm 50. The parts of P7 entering arm 50 retains itsoriginal phase shift, while the portion of P9 crossing over in thehybrid has a 90 phase shift. There are now two electromagnetic waves ofequal power and of identical phase in arm 50 which reinforce each otherand travel as one wave. Therefore all the reflected power will traveltowards arm 50.

If water load 55 is used, the power in arm 50 is absorbed and lost tothis apparatus. However, it is preferable to use the reflector 53, whichreflects this power back through the hybrid. This second reflected poweris directed against the edge of the lumber and the power reflected fromthe latter is subjected to the same conditions in the hybrid as theoriginal reflected power, with the exception that the power reflectedthe second time by the lumber is directed towards the microwavegenerator, but it is so small that it is not worth considering, .and themicrowave generator does not need protection against it. For example, if7% of the original power is reflected by the board, because of the angleof the board relative to this power, 7% of the original power isreflected by reflector 53 and when this comes up against the boardagain, only 7% of that or approximately .5% of the original power isreflected again.

If desired, an auxiliary wavelength system can be connected to arm 50 ofhybrid 26 instead of water load 55 or reflector 53, so as to use thereflected power for drying purposes.

The combination of a 4-port hybrid, parallel waveguides and angledlumber flow has the advantages that the microwave generator is protectedagainst reflected microwave energy, and practically all of the generatedpower is directed to and into the lumber. In the first instanceapproximately 93% of the power enters the lumber, and approximately 93%of the reflected power directed back to the wood enters the latter.Another advantage of this apparatus is that an auxiliary waveguidesystem may be connected by hybrid arm 50 in place of reflector 53 sothat the original power reflected by the wood in waveguides 23 and 24would be utilized in this auxiliary system.

What is claimed is:

1. Waveguide apparatus for drying moisture-laden dielectric material bymicrowave energy, comprising a set of parallel waveguides each havinglongitudinal centerline, a microwave generator coupled to thewaveguides, slots in said waveguides through which material to be driedcan pass to be exposed to microwave energy from the generator, saidslots being arranged so that the path of travel of the material is at anangle of less than to said centerline, and stop means between thegenerator and the waveguides for preventing microwave energy reflectedby the material from reaching the generator.

2. Waveguide apparatus as claimed in claim 1 in which said stop means isa 4-port hybrid which acts as coupling means between the microwavegenerator and said waveguides.

3. Waveguide apparatus as claimed in claim 2 including microwave energyabsorption means coupled to one of the ports of said hybrid.

4. Waveguide apparatus as claimed in claim 2 including a reflectorcoupled to one of the ports of said hybrid for reflecting microwaveenergy back to said waveguides.

5. Waveguide apparatus comprising a hybrid having a common centralsection and a longitudinal centerline, a first pair of arms projectingfrom one end of the central section on opposite sides of the centerline,waveguides coupled to the first pair of arms, a second pair of armsprojecting from an opposite end of the central section on opposite sidesof the hybrid centerline, a microwave generator coupled to one arm ofsaid second pair of arms so that microwave energy therefrom is directedthrough both of the arms of the first pair, and slots in said waveguidesthrough which material to be dried can pass to be exposed to saidmicrowave energy, said slots being arranged so that the path of travelof the materital is at an angle of less than 90 to said centerline.

'6. Waveguide apparatus as claimed in claim 5 including microwave energyabsorption means coupled to the other arm of said second pair of arms.

7. Waveguide apparatus as claimed in claim 5 including a reflectorcoupled to the other arm of said second pair of arms for reflectingmicrowave energy back through said arms of the first pair of arms.

References Cited UNITED STATES PATENTS I. V. TRUHE, Primary Examiner H.D. JAEGER, Assistant Examiner US. Cl. X.R. 34-1

